diff --git a/hosts/modules/hal/pine64-pinephone-pro/default.nix b/hosts/modules/hal/pine64-pinephone-pro/default.nix
index e232cff81..d352e2093 100644
--- a/hosts/modules/hal/pine64-pinephone-pro/default.nix
+++ b/hosts/modules/hal/pine64-pinephone-pro/default.nix
@@ -68,6 +68,10 @@ in
         name = "rk3399-pinephone-pro-sound";
         dtsFile = ./rk3399-pinephone-pro-sound.dtso;
       }
+      {
+        name = "rk3399-pinephone-pro-modem";
+        dtsFile = ./rk3399-pinephone-pro-modem.dtso;
+      }
     ];
 
     # this seems to not actually be necessary (unless maybe my *overlay* #include's stuff)
diff --git a/hosts/modules/hal/pine64-pinephone-pro/rk3399-pinephone-pro-modem.dtso b/hosts/modules/hal/pine64-pinephone-pro/rk3399-pinephone-pro-modem.dtso
new file mode 100644
index 000000000..cdcd3fb62
--- /dev/null
+++ b/hosts/modules/hal/pine64-pinephone-pro/rk3399-pinephone-pro-modem.dtso
@@ -0,0 +1,132 @@
+// checked against pine64-org/linux 6.6-20231104-22589
+//   i.e. the kernel used by postmarketOS
+// schematics for reference: <https://files.pine64.org/doc/PinePhonePro/PinephonePro-Schematic-V1.0-20211127.pdf>
+//
+// modem gets power on these lines:
+// VCC3V3_S3
+//   - comes from RK818's LDO8; already enable in mainline DTS
+// VCC_4G
+//   - generated by feeding VCC_SYS through some gates
+//     - requires 4G_PWR (RK3399:GPIO4_C7) to be high
+//     - requires hardware kill-switch to be enabled
+//   - VCC_SYS is surely already enabled (this is the main power rails)
+// VCC_4G_5V
+//   - generated by feeding VCC5V0_SYS through some gates
+//     - requires 4G_5V_ON (RK3399:GPIO1_C7) to be high
+//   - VCC5V0_SYS appears to come from RK818 BOOST_1/BOOST_2
+//     alternately supplied by Type-C VCONN_12,VCONN_13
+//
+// modem is connected to RK3399 I2C1, via level shifter
+// - pine64-org/linux specs i2c1 as 400kHz, which is precisely what RK818 (the PMIC) specs in its datasheet
+//   however, RK818 isn't documented as being on any I2C line in the PPP schematics.
+// - I2C1 frequency would be limited by EG25 modem itself
+//   and by the level shifter
+//   and by the ALC5640 codec (system's)
+//   - max I2C clock is 400kHz
+//   - max rise: 300ns
+//   - max fall: 300ns
+//   and by the ALC5616 codec (modem's)
+//   and by the AXT530124 MIPI controllers for the front and rear cameras
+//   and by the OV8858 front camera (via MIPI)  (maybe?)
+//   and by the IMX258 rear camera (via MIPI)   (maybe?)
+// - I2C1 is pulled up by VCC1V8_CODEC (4.7kOhm)
+
+// for RK_PC7, other pins
+#include <dt-bindings/pinctrl/rockchip.h>
+// for GPIO_ACTIVE_HIGH
+#include <dt-bindings/gpio/gpio.h>
+
+/dts-v1/;
+/plugin/;
+
+/ {
+        /* ensure this overlay applies only to the correct board */
+        compatible = "pine64,pinephone-pro";
+};
+
+&{/} {
+        vcc_4g_5v: vcc-4g-5v {
+                compatible = "regulator-fixed";
+                enable-active-high;
+                gpio = <&gpio1 RK_PC7 GPIO_ACTIVE_HIGH>;
+                pinctrl-names = "default";
+                pinctrl-0 = <&vcc_4g_5v_en>;
+                regulator-name = "vcc_4g_5v";
+                regulator-min-microvolt = <5000000>;
+                regulator-max-microvolt = <5000000>;
+                vin-supply = <&vcc5v0_sys>;
+                regulator-always-on;
+        };
+
+        vcc_4g: vcc-4g {
+                compatible = "regulator-fixed";
+                enable-active-high;
+                gpio = <&gpio4 RK_PC7 GPIO_ACTIVE_HIGH>;
+                pinctrl-names = "default";
+                pinctrl-0 = <&vcc_4g_en>;
+                regulator-name = "vcc_4g";
+                // 3.8V is copied from pine64-org/linux.
+                // i think vcc_4g should match whatever vcc_sys is set to (it's input).
+                // and vcc_sys would seem to be the battery voltage?
+                //   (even when powered via USB)
+                regulator-min-microvolt = <3800000>;
+                regulator-max-microvolt = <3800000>;
+                vin-supply = <&vcc_sys>;
+                regulator-always-on;
+        };
+
+        vcc5v0_sys: vcc5v0-host-regulator {
+                compatible = "regulator-fixed";
+                regulator-name = "vcc5v0_sys";
+                regulator-min-microvolt = <5000000>;
+                regulator-max-microvolt = <5000000>;
+                regulator-always-on;
+                regulator-boot-on;
+                vin-supply = <&boost_otg>;
+
+                regulator-state-mem {
+                        regulator-on-in-suspend;
+                };
+        };
+};
+
+&pinctrl {
+        modem {
+                vcc_4g_5v_en: vcc-4g-5v-en-pin {
+                        rockchip,pins = <1 RK_PC7 RK_FUNC_GPIO &pcfg_pull_none>;
+                };
+
+                vcc_4g_en: vcc-4g-en-pin {
+                        rockchip,pins = <4 RK_PC7 RK_FUNC_GPIO &pcfg_pull_none>;
+                };
+        };
+};
+
+// rk3399-pinephone-pro-sound sets this 'okay', but do that here also to avoid implicit dependencies.
+&i2c1 {
+        status = "okay";
+        // N.B.: these values are duplicated in rk3399-pinephone-pro-sound.dtso
+        clock-frequency = <400000>;
+        i2c-scl-rising-time-ns = <300>;
+        i2c-scl-falling-time-ns = <15>;
+};
+
+&rk818 {
+        regulators {
+                boost_otg: DCDC_BOOST {
+                        regulator-name = "boost_otg";
+                        regulator-always-on;
+                        regulator-boot-on;
+                        regulator-min-microvolt = <5000000>;
+                        regulator-max-microvolt = <5000000>;
+                        regulator-state-mem {
+                                regulator-on-in-suspend;
+                        };
+                };
+
+                // probably not strictly required here
+                otg_switch: OTG_SWITCH {
+                        regulator-name = "otg_switch";
+                };
+        };
+};